if you have an Enterprise license, you have a set number of 'authorized contacts' that you can associate with your account as part of your Support contract. one or more of these authorized contacts can be designated as a 'Portal Admin'. if you are logged in as a "Portal admin" to the Support Portal, you can add authorized contacts up to your limit.
an important point: those users must already exist as part of your company's account. if they don't, as lukejadamec points out, you should contact Splunk Support.